Flutter Drawer Widget - change Scaffold.body content

You can use this for body of Scaffold:

        body: _getDrawerItemWidget(_selectedDrawerIndex));

and then in the main or state class do like so:

class HomePageState extends State<HomePage> {
  int _selectedDrawerIndex = 0;

  _getDrawerItemWidget(int pos) {
    switch (pos) {
      case 0:
        return new MainFragment();
      case 1:
          return new CardMgmt();
}
}
}

CardMgmt and other classes in switch case have their own build method that will return a Widget and it will be place in body of your scaffold. Also they can be in the same dart file or even another file.
But I have another problem:
What to do if instead of drawer items, we have a button in another class like CardMgmt and we want it to return a Widget and update page?

That's a very good solution from Aziza! Thank you. It works! I'd just want to add that it can also work for the whole body by replacing it with a widget state:

...
class _TestPageState extends State<TestPage> {
  String text = "Initial Text";
  Widget widgetForBody = SomeWidgetFromClass();
  ...
  ...
        children: <Widget>[
          new Container(child: new DrawerHeader(child: new Container())),
          new Container (
            child: new Column(
                children: <Widget>[
                  new ListTile(leading: new Icon(Icons.info),
                      onTap:(){
                          setState((){
                            widgetForBody = AnotherWidgetFromClass();
                          });
                      }
                  ),
                  new ListTile(leading: new Icon(Icons.save),
                      onTap:(){
                        setState((){
                          widgetForBody = YetAnotherWidgetFromClass();
                        });
                      }
                  ),
...

You can simply change the state of the content of your interest by interacting with your Drawer items like this:

Here is the relevant code for this example:

class TestPage extends StatefulWidget {
  @override
  _TestPageState createState() => new _TestPageState();
}

class _TestPageState extends State<TestPage> {
  String text = "Initial Text";
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        drawer: new Drawer(
          child: new ListView(
            children: <Widget>[
              new Container(child: new DrawerHeader(child: new Container())),
              new Container (
                child: new Column(
                    children: <Widget>[
                      new ListTile(leading: new Icon(Icons.info),
                          onTap:(){
                              setState((){
                                text = "info pressed";
                              });
                          }
                      ),
                      new ListTile(leading: new Icon(Icons.save),
                          onTap:(){
                            setState((){
                              text = "save pressed";
                            });
                          }
                      ),
                      new ListTile(leading: new Icon(Icons.settings),
                          onTap:(){
                            setState((){
                              text = "settings pressed";
                            });
                          }
                      ),

                    ]
                ),
              )
            ],
          ),
        ),
        appBar: new AppBar(title: new Text("Test Page"),),
        body: new Center(child: new Text((text)),
        ));
  }
}